1. Field of the Invention
The present invention relates to diffractive optical elements and, more particularly, to a diffractive optical element having such a grating structure that concentrates a plurality of wavelengths or a predetermined band of wavelengths of light on a particular order (or design order). Still more particularly, it relates to optical systems using such diffractive optical elements.
2. Description of Related Art
Heretofore, the optical systems have been corrected for chromatic aberrations by combining two glass materials (lenses) of different dispersion.
Unlike this conventional method of reducing the chromatic aberrations, a novel method of further reducing the chromatic aberrations by using a diffractive optical element in which a diffraction grating having a diffracting function is provided on a lens surface or in part of the optical system is disclosed in, for example, SPIE Vol. 1354 xe2x80x9cInternational Lens Design Conference xe2x80x9d (1990), Japanese Laid-Open Patent Applications No. Hei 4-213421 and No. Hei 6-324262 and U.S. Pat. No. 5,044,706.
The above novel method has been developed by utilizing a physical phenomenon that the refractive surface and the diffractive surface in an optical system produce chromatic aberrations for the rays of light of a certain reference wavelength in respective opposite directions.
Moreover, such a diffractive optical element can be made to behave like an aspherical lens by changing the periodic structure thereof, giving a great advantage of reducing many aberrations.
Here, the actions of light passing through the refractive surface and the diffractive surface are compared. For the refractive surface, one beam after being refracted even remains one in number. For the diffractive surface, one beam, when diffracted, breaks up into a number of beams in diffraction orders.
Thus, if the diffractive optical element is used as a lens system, there is need to decide the grating structure so that a light beam of the usable wavelength region concentrates in a particular order (hereinafter also referred to as xe2x80x9cdesignxe2x80x9d order). In a situation when light concentrates on the particular order, the intensities of light beams of the other diffraction orders become low. If the intensity is xe2x80x9c0xe2x80x9d, the corresponding diffracted light can be regarded as not existing.
For that purpose, the diffraction efficiency for light beams in the design order must be high enough. In another situation when the diffracted rays in the other orders than the design order are present, they focus themselves at different places than that at which the rays in the design order do, becoming light to produce flare.
In an optical system employing the diffractive optical element, it is, therefore, of great importance to fully consider even the spectral distribution of diffraction efficiencies in the design order and the behavior of the rays in the other orders.
FIG. 12 shows a mono-layer diffractive optical element 1 in which a diffraction grating 3 is formed on a substrate 2, constituting part of an optical system. FIG. 13 shows characteristic curves of the diffractive optical element 1 for selected orders of the diffraction efficiency against wavelengths.
The values of the diffraction efficiency are in percentage of the amount of diffracted light of each wavelength to the total amount of transmitted light. The reflected light from the grating boundary or the like is not taken into account in the evaluation.
As the optical material of the diffractive optical element 1, use is made of a plastic material named xe2x80x9cPMMAxe2x80x9d (nd=1.4917, vd=57.4). The grating thickness xe2x80x9cdxe2x80x9d is determined to be 1.07 xcexcm. In FIG. 13, the abscissa axis represents wavelength and the ordinate axis represents diffraction efficiency.
The diffractive optical element 1 is so designed that the diffraction efficiency is highest at a wavelength of 530 nm in the first order (solid line in FIG. 13). That is, the design order is the first order.
Further, the diffraction efficiencies in the next orders to the first one (or (1xc2x11)st orders, namely, the zero order and the second order) are also depicted for comparison. As shown in FIG. 13, it is in the design order that the diffraction efficiency takes a highest value at a certain wavelength (hereinafter referred to as xe2x80x9cdesignxe2x80x9d wavelength) and falls off gradually at other wavelengths.
In the arrangement described above, the lowered portion of the diffraction efficiency in the design order becomes the diffracted rays in the other orders, that is, flare. Also, if a plurality of diffraction gratings are used, the lowering of the diffraction efficiency at the other wavelengths than the design wavelength leads to reduction of the transmittance.
An arrangement that can diminish the lowering of the diffraction efficiency has been proposed in Japanese Laid-Open patent Application No. Hei 10-133149. The diffractive optical element suggested in this proposal is shown in FIG. 14, where two diffraction gratings 4 and 5 are applied on a substrate 2 in superimposed relation to form a diffraction grating of laminated structure in cross-section. The refractive indices and dispersion characteristics of the materials of the two layers and the grating thickness of each layer are made optimal to realize a high diffraction efficiency over the whole visible spectrum. FIG. 15 is a graph for explaining the spectral characteristics of the diffractive optical element 1 shown in FIG. 14.
Another arrangement that can diminish the lowering of the diffraction efficiency is proposed in Japanese Laid-Open Patent Application No. Hei 9-127322. As shown in FIG. 16, three diffraction gratings 4, 21 and 5 are made up by optimally selecting three different kinds of material and two different grating thicknesses d1 and d2, and are laid adjacent to each other on the substrate 2 with their grooves distributed in equal pitches, thus realizing a high diffraction efficiency throughout the entire range of visible spectrum, as shown in FIG. 17.
In the prior known diffractive optical elements described above, the detailed specification is available about the materials and grating thicknesses with which to form the relief pattern (hereinafter also referred to as the xe2x80x9cdiffraction grating surfacexe2x80x9d). However, concerning what values to use in the grating pitch and the total sum of the grating thicknesses including the interval between the two gratings (diffraction gratings), only the condition of the Q value satisfying the thin type of diffraction grating is explained.
However, as a plurality of diffraction gratings are laminated to reduce the lowering of the diffraction efficiency over the entire range of visible spectrum, a number of design parameters have to be taken into account. Among all the combinations of the factors, the ones disclosed in the above-cited documents are not always the best in some cases. Although will be more fully described later, for example, the grating pitch is too much small, and the interval between the two diffraction gratings (the thickness d3 in FIG. 16) is too much large.
Further, in the prior known diffractive optical elements described above, with regard to the grating pitch in each relief pattern, it is only suggested that all the pitches are made equal to one another and the corresponding grooves are arranged in confronting relation, and what are concerned with these are only illustrated. In other words, a premise is made such that the grating edges of the two relief patterns are aligned to each other.
The present invention is applied to the diffractive optical element having two or more diffraction gratings of different kinds appropriately stratified on a substrate, and has an aim to obtain a high diffraction efficiency even when the grating pitch is small, or when the grating thickness is large, thus making it possible to effectively suppress flare or the like.
According to a first aspect of the present invention, there is provided a diffractive optical element, comprising a plurality of diffraction gratings which differ in dispersion from each other and are laminated while being spaced to enhance diffraction efficiency for a particular order (design order) throughout an entire usable wavelength region, wherein edges of at least part of corresponding grating portions of the plurality of diffraction gratings are shifted from each other in a direction of arrangement of grating portions of each diffraction grating.
According to a second aspect of the present invention, there is provided a diffractive optical element, comprising a plurality of diffraction gratings which differ in dispersion from each other and are laminated while being spaced to enhance diffraction efficiency for a particular order (design order) throughout an entire usable wavelength region, wherein grating pitches of at least part of corresponding grating portions of the plurality of diffraction gratings are different from each other.
According to a third aspect of the present invention, as applied to the diffractive optical element in the first or second aspect, a grating pitch of each of the plurality of diffraction gratings is gradually varied, and shifting of edges between corresponding grating portions of the plurality of diffraction gratings becomes larger accordingly as the grating pitch of each grating portion becomes smaller.
According to a fourth aspect of the present invention, as applied to the diffractive optical element in the first or second aspect, where n1L and n2L are refractive indices for an arbitrary wavelength xcex on a light-entrance side and a light-exit side, respectively, of a material of the L-th diffraction grating, when counted from a substrate, of the plurality of diffraction gratings, xcex8L is an angle an incident ray makes with a normal line of a grating, P is a grating pitch, dL is a grating thickness of the L-th diffraction grating, and DL is a distance between the L-th diffraction grating and the (L+1)st diffraction grating, a positional shift xcex94xc3x97L between edges of corresponding grating portions of the (L+1)st diffraction grating and the L-th diffraction grating satisfies the following condition:
0 less than xcex94xc3x97Lxe2x89xa62((dL/2)+DL)*tan xcex1L+1 
xcex1L+1=sinxe2x88x921((n1L*sinxcex8Lxe2x88x92(n1Lxe2x88x92n2L)d1/P)/n2L). 
According to a fifth aspect of the present invention, as applied to the diffractive optical element in the first aspect, the diffractive optical element has a plurality of areas in a light-transmitting plane, and edges of corresponding grating portions of laminated diffraction gratings in part of the plurality of areas are shifted from each other.
According to a sixth aspect of the present invention, as applied to the diffractive optical element in the second aspect, the diffractive optical element has a plurality of areas in a light-transmitting plane, and grating pitches of corresponding grating portions of laminated diffraction gratings in part of the plurality of areas are different from each other.
According to a seventh aspect of the present invention, as applied to the diffractive optical element in the first aspect, shifting of edges between corresponding grating portions of the plurality of diffraction gratings is realized by changing the width of a grating pitch at particular locations of the grating portions.
According to an eighth aspect of the present invention, as applied to the diffractive optical element in the second aspect, a difference between grating pitches of corresponding grating portions of the plurality of diffraction gratings is realized by changing the width of a grating pitch at particular locations of the grating portions.
According to a ninth aspect of the present invention, as applied to the diffractive optical element in the first or second aspect, the plurality of diffraction gratings include at least one diffraction grating a direction of increase of a grating depth of which is different from that of the other diffraction gratings.
According to a tenth aspect of the present invention, as applied to the diffractive optical element in the first or second aspect, the usable wavelength region is a visible spectrum.
According to an eleventh aspect of the present invention, as applied to the diffractive optical element in the first or second aspect, the first diffraction grating, when counted from a substrate, of the laminated plurality of diffraction gratings is made from the same material as that of the substrate.
According to a twelfth aspect of the present invention, as applied to the diffractive optical element in the first or second aspect, the plurality of diffraction gratings consist of two diffraction gratings having respective grating portions edges or pitches of which are shifted or different from each other, the two diffraction gratings confronting each other across an air layer.
According to a thirteenth aspect of the present invention, there is provided a diffractive optical element, comprising a plurality of diffraction gratings which differ in dispersion from each other and are laminated while being spaced, wherein, with respect to at least two wavelengths, a maximum optical path length difference is made equal to integer times each of the wavelengths, and wherein edges of at least part of corresponding grating portions of the plurality of diffraction gratings are shifted from each other in a direction of arrangement of grating portions of each diffraction grating.
According to a fourteenth aspect of the present invention, there is provided a diffractive optical element, comprising a plurality of diffraction gratings which differ in dispersion from each other and are laminated while being spaced, wherein, with respect to at least two wavelengths, a maximum optical path length difference is made equal to integer times each of the wavelengths, and wherein grating pitches of at least part of corresponding grating portions of the plurality of diffraction gratings are different from each other.
According to a fifteenth aspect of the present invention, as applied to the diffractive optical element in the thirteenth or fourteenth aspect, a grating pitch of each of the plurality of diffraction gratings is gradually varied, and shifting of edges between corresponding grating portions of the plurality of diffraction gratings becomes larger accordingly as the grating pitch of each grating portion becomes smaller.
According to a sixteenth aspect of the present invention, as applied to the diffractive optical element in the thirteenth or fourteenth aspect, where n1L and n2L are refractive indices for an arbitrary wavelength xcex on a light-entrance side and a light-exit side, respectively, of a material of the L-th diffraction grating, when counted from a substrate, of the plurality of diffraction gratings, xcex8L is an angle an incident ray makes with a normal line of a grating, P is a grating pitch, dL is a grating thickness of the L-th diffraction grating, and DL is a distance between the L-th diffraction grating and the (L+1)st diffraction grating, a positional shift xcex94xc3x97L between edges of corresponding grating portions of the (L+1)st diffraction grating and the L-th diffraction grating satisfies the following condition:
xe2x80x830 less than xcex94xc3x97Lxe2x89xa62((dL/2)+DL)*tan xcex1L+1 
xcex1L+1=sinxe2x88x921((n1L*sinxcex8Lxe2x88x92(n1Lxe2x88x92n2L)d1/P)/n2L). 
According to a seventeenth aspect of the present invention, as applied to the diffractive optical element in the thirteenth aspect, the diffractive optical element has a plurality of areas in a light-transmitting plane, and edges of corresponding grating portions of laminated diffraction gratings in part of the plurality of areas are shifted from each other.
According to an eighteenth aspect of the present invention, as applied to the diffractive optical element in the fourteenth aspect, the diffractive optical element has a plurality of areas in a light-transmitting plane, and grating pitches of corresponding grating portions of laminated diffraction gratings in part of the plurality of areas are different from each other.
According to a nineteenth aspect of the present invention, as applied to the diffractive optical element in the thirteenth aspect, shifting of edges between corresponding grating portions of the plurality of diffraction gratings is realized by changing the width of a grating pitch at particular locations of the grating portions.
According to a twentieth aspect of the present invention, as applied to the diffractive optical element in the fourteenth aspect, a difference between grating pitches of corresponding grating portions of the plurality of diffraction gratings is realized by changing the width of a grating pitch at particular locations of the grating portions.
According to a twenty-first aspect of the present invention, as applied to the diffractive optical element in the thirteenth or fourteenth aspect, the plurality of diffraction gratings include at least one diffraction grating a direction of a grating portion of which is different from that of the other diffraction gratings.
According to a twenty-second aspect of the present invention, as applied to the diffractive optical element in the thirteenth or fourteenth aspect, the two wavelengths are included in a visible spectrum.
According to a twenty-third aspect of the present invention, as applied to the diffractive optical element in the thirteenth or fourteenth aspect, the first diffraction grating, when counted from a substrate, of the laminated plurality of diffraction gratings is made from the same material as that of the substrate.
According to a twenty-fourth aspect of the present invention, as applied to the diffractive optical element in the thirteenth or fourteenth aspect, the plurality of diffraction gratings consist of two diffraction gratings having respective grating portions edges or pitches of which are shifted or different from each other, the two diffraction gratings confronting each other across an air layer.
According to a twenty-fifth aspect of the present invention, as applied to the diffractive optical element in one of the first to twenty-fourth aspects, the diffraction efficiency in a particular order is made not less than 95% in a wavelength range of 450 nm to 650 nm.
Further, there is provided an image forming optical system or an observing optical system using the diffractive optical element in one of the first to twenty-fifth aspects.
These and further aspects and features of the present invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.